1. Field of the Invention
The present invention relates to a cathode active material and a battery employing the same, and more particularly, to an organopolysulfide cathode active material, the particle size of which is easy to process, and which has excellent electrochemical utilization efficiency and capacity, and to a lithium battery employing the same.
2. Description of the Related Art
As the use of portable devices has rapidly increased, there has been a gradually increasing necessity for high-performance secondary batteries. Also, the miniaturization of such portable devices has fueled the desire for development of light-weight, high-energy density secondary batteries.
In conventional secondary batteries, although light-weight and high-capacity performance has been achieved to an extent, environmental problems due to the use of heavy metal arise. Thus, secondary batteries using environmentally benign materials have become keenly required.
In attaining high-capacity batteries, one of the critical issues is to develop low equivalent weight electrode active materials. Here, the electrode active materials must have sufficient ionic conductivity and high reversibility of an oxidation/reduction reaction, as well as excellent thermal and chemical stability. Also, the electrode materials must be reasonably inexpensive, widely available, non-toxic, and easy to process.
As cathode active materials having the above-described required characteristics, the following organo-sulfur compounds are known.
Polyplus Battery Company, Inc. has developed an organosulfur cathode active material represented by general formula (R(S)y)n, wherein R is an organic group having 1 to 20 carbon atoms, y is an integer from 1 to 6 and n is an integer from 2 to 20. The battery utilizing this cathode material has a poor lifetime characteristic, although it exhibits excellent energy density.
Organosulfur cathode active materials disclosed in U.S. Pat. Nos. 5,686,201 and 5,532,077, although having an excellent energy density, have a poor cycle characteristic and a low electric capacity of approximately 67%, that is, a necessity of charging 150% of a discharge electric capacity.
Known cathode active materials developed by Moltech Corporation include (CSx)n, wherein x is 1.7-2.3 and n is an integer from 2 to 20, and (C2Sx)n wherein x is 1-10 and n is greater than or equal to 2 (U.S. Pat. No. 5,529,860). Those cathode active materials have theoretically high specific capacity but have poor utilization efficiency, low reversibility of the oxidation/reduction reaction, that is, charging/discharging reaction, and relatively low electric capacity due to a restricted amount of sulfur participating in the electrochemical reaction, resulting from the formation of an insulating sulfur film on an electrode portion.